What about optimisation power of $x^{\prime }$ as a measure of outcome that have utility greater than the utility of $x^{\prime }$? 

Let $U_{x^{\prime }}$ be the set of outcome with utility greater than $x^{\prime }$ according to utility function $u$:

The set  $U_{x^{\prime }}$  is invariant under translation and non-zero rescaling of the utility function $u$ and we define the optimisation power of the outcome $x$' according to utility function $u$ as:

This does not suffer from comparing w.r.t a worst case and seem to satisfies the same intuition as the original OP definition while referring to some utility function.

This is in fact the same measure as the original optimisation power measure with an order given by the utility function

Thanks for all the useful links! I'm also always happy to receive more feedback.

I agree that the sense in which I use metaethics in this post is different from what academic philosophers usually call metaethics. I have the impression that metaethics, in academic sense, and metaphilosophy are somehow related. Studying what morality itself is, how to select ethical theories and what is the process behind ethical reasoning seems not independent. For example if moral nihilism is more plausible then it seems to be less likely that there is some meaningful feedback loop to select ethical theories or that there is such a meaningful thing as a ‘good’ ethical theory (at least in an observer independent way) . If moral emotivism is more plausible then maybe reflecting on ethics is more like emotions rationalisation, e.g. typically expressing in a sophisticated way something that just fundamentally means ‘boo suffering’. In that case having better understanding of metaethics in the academic sense seems to bring some light to a process that generates ethical theories, at least in humans.